Process for forming plasticized polyvinyl butyral sheet

ABSTRACT

Plasticized polyvinyl butyral sheet optionally containing a gradient color band is formed by continuously extruding a sheet having a predetermined region across its width formed from melt issuing from a die at an increased mass flow rate in comparison with other portions of the melt forming the rest of the sheet and then forwardly distorting such sheet immediately after forming by preferentially stretching the side containing the predetermined region to form an arcuate shape of substantially uniform thickness. Arcuately shaped sections are then cut from the sheet to form discrete, shaped interlayer blanks having improved thickness uniformity and dimensional stability providing improved performance when used in laminated safety glass.

CROSS REFERENCE TO RELATED APPLICATIONS

1. "Process For Forming Shaped Interlayer Blanks", D. P. Bourcier, R. A.Esposito, Ser. No. 121,197 filed Nov. 17, 1987, now Pat. No. 4,808,357

2. "Apparatus For Forming Shaped Interlayer Blanks", D. P. Bourcier, R.A. Esposito, Ser. No. 121,546 filed Nov. 17, 1987, now Pat. No.4,768,939.

BACKGROUND OF THE INVENTION

This invention relates to forming thermoplastic interlayer sheeting andmore particularly to a method for forming discrete, shaped blanks ofplasticized polyvinyl butyral suitable for laminated safety glasswindshields.

Thin sheet formed of plasticized polyvinyl butyral is well known as aninterlayer in laminated safety glass finding application in windows suchas penetration-resistant automobile windshields. A very well knownproblem in view of many patents dealing with it since the first in theUnited States issued in 1952, is the performance deficiency in theinterlayer when used in a specially curved and angled windshield. Morespecifically, sheet in such applications has traditionally been shapedin association with glass laminating downstream of sheet forming bystretching into a shape to match the desired windshield configuration.Special problems arise when the interlayer has a colored band along onemargin which is intended to reduce glare from the sun when in place inthe windshield. The band is usually graduated in color intensity withthe greatest color along the upper peripheral portion of the windshieldwhich then gradually diminishes to an almost imperceptible cutoff lineat the lower edge of the band. When such gradient band windshield hashorizontal and vertical curvature, as is usual with modern wrap aroundauto windshields, it has been necessary to stretch each sheet section toan arcuate shape before associating it with the glass so that afterlamination the cut-off line of the band is parallel to the upper edge ofthe windshield. This amount of initial stretching compensates for theinitial lack of curvature of the color band which if unstretched wouldproduce a cut-off line not parallel to the upper edge of a curved,wrap-around windshield.

As well delineated in the prior art, for example col. 2, lines 3-28 ofU.S. Pat. No. 4,244,997, stretching of sheet after its formationadversely affects its subsequent performance in a windshield. Morespecifically, differential stretching inherently results in thicknessreduction and a buildup of strain unless relieved. Non-uniform thicknesstranslates to variable impact resistance in the windshield, suchresistance being lower in the area of reduced thickness. Uneven strainresults in variable shrink-back and reduction in peripheral size of thesheet during handling before trapping it in place between layers duringlamination. When this occurs the shaped blank must be discarded if itreverts to a smaller size than the glass sheets with which it is beinglaminated. Reheating sheet previously conventionally formed with a roughsurface to facilitate deairing during lamination can prematurelyundesirably reduce such roughness rendering it more susceptible to stacksticking in storage and less effective in allowing air to escape duringlamination. Further, reheating and stretching sheet previously havingpredetermined levels of moisture carefully incorporated therein to helpcontrol its level of adhesion to adjoining glass layers can cause suchmoisture to flash out of the sheet. It would be desirable to provide asystem for forming interlayer windshield blanks reducing or eliminatingthe foregoing plethora of shortcomings.

SUMMARY OF THE INVENTION

Now improvements have been made which overcome or diminish prior artshortcomings with respect to interlayer blanks for laminated safetyglass windows.

Accordingly, it is a principal object of this invention to provide animproved method for forming interlayer blanks of plasticized polyvinylbutyral usable with glass in automotive windshields.

Another object is to provide such a method which uses heat in thepolymer for initially forming the sheet to facilitate its shaping to fita curved windshield.

An additional object is to provide during its manufacture an on-linemethod of shaping plasticized polyvinyl butyral sheeting into a curvedconfiguration matching the contour of the windshield of which it willeventually be a component.

A further object is to provide a cut blank for a windshield or abuilding window having improved thickness uniformity and dimensionalstability providing improved impact performance in a laminated glassassembly.

Other objects of this invention will in part be obvious and will in partappear from the following description and claims.

These and other objects are accomplished by providing a process forforming plasticized polyvinyl butyral sheet which comprises continuouslyforcing plasticized polyvinyl butyral melt through a slot to form asheet, the mass flow rate of melt through one region of the slot beinggreater than through the remainder of the slot; and distorting the sheetinto a curved shape of substantially uniform thickness having adifferent curvature along each edge while the melt continues to issuefrom the slot, the edge of greater curvature being formed from theregion of the sheet formed from the melt issuing through the slot at thegreater mass flow rate.

Though applicable to the formation of clear interlayer blanks, theprocess preferably includes the step of incorporating a colored streamof plasticized polyvinyl butyral into the melt upstream of the slot toproduce a sheet having a gradient color band extending along one side.

BRIEF DESCRIPTION OF THE DRAWINGS

In describing the overall invention, reference will be made to theaccompanying drawings wherein:

FIG. 1 is a schematic, elevational view of a system embodying theinvention;

FIGS. 2 and 3 are each plan views of successive parts of the system ofFIG. 1;

FIG. 4 is a schematic view in partial section along 4--4 of FIG. 2; and

FIG. 5 is a sectional view along 5--5 of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, apparatus, collectively identified as 10,is shown in FIGS. 1-3 for forming discrete, shaped interlayer blanks.Apparatus 10 comprises extrusion shaping die 11 having slot 16 (FIG. 4)at its outlet for forming sheet 12 from plasticized polyvinyl butyral,means 14 (FIGS. 4, 5) associated with die 11 for causing a portion ofthe melt forming sheet 12 to exit slot 16 at a greater mass flow ratethan remaining melt portions simultaneously exiting slot 16, means 18(FIGS. 1 and 2) adjacent die 11 for quenching sheet 12 immediately afterforming, frusto-conical pull rolls 20 (FIG. 4) for forwardly shapingsheet 12 into arcuate form, and cutting means 22 (FIGS. 1 and 2) beyondrolls 20 for repeatedly, successively cyclically severing quadrilateralsections out of shaped sheet 12 to form on each occasion a discrete,shaped, interlayer blank 24. (FIG. 1).

Means 18 for quenching sheet 12 comprises a cooling medium 26, which maybe water, within tank 28 (FIGS. 1 and 4) below die 11 in whichvertically lowermost frusto-conical pull roll 20a is at leastsemi-immersed. In place of or in addition to tank 28 and medium 26, acoolant conventionally controlled at a desired temperature could becirculated continuously through one or both rolls 20. In the illustratedembodiment, roll 20a has a conventional uncovered metal surface but, ifdesired, it may be covered with an appropriate material such as rubberor otherwise treated to promote frictional engagement with sheet 12.Roll 20a is rotatably mounted for turning movement by a conventionaldrive means operatively associated with drive shaft 98, not shown. Roll20a peripherally engages (prior to conducting the process as will bedescribed) cooperating, upper frustoconical idler roll 20b (though itcould also be driven), both such rolls defining nip 97 therebetweenthough which passes sheet 12 as will be described.

Die 11 (FIG. 4) is on the discharge end of a conventional extrusionsystem, not shown, comprising one or two screws turning within a casingfor advancing plasticized polyvinyl butyral melt through cylindricalmanifold 30 toward and through slot 16. Substantially parallel lips 32,34 each have a preferably smooth land surface and together delimit anddefine slot 16. Approach passage 40 intercommunicates at one end withmanifold 30 and converges at its forward end to slot 16.

Die 11 preferably includes a generally torpedo-shaped probe 42 (FIG. 4)within and affixed via supports 44a and 44b to the walls of manifold 30.Probe 42 is eccentrically offset just forward of manifold axis 45 towardapproach passage 40 and has its long axis parallel to slot 16. Probe 42contains an extrusion slot, not shown, parallel with its long axis forincorporating, upstream of slot 16, a colored stream of melt into themain melt stream forming sheet 12, to provide gradient color band 46(FIG. 2) in and extending along one side of sheet 12. Further details ofprobe 42 are described in U.S. Pat. No. 4,316,868, col. 2, line 60through col. 6 line 7, the content of which is incorporated herein byreference.

Means 14 (FIGS. 4, 5) associated with-die 11 for causing duringextrusion a portion of the melt to exit slot 16 at a greater mass flowrate than remaining portions of the melt, comprises metal choke bar 47slidably mounted within a slot 48 which extends crosswise across thefull width of and parallel to slot 16 of die 11. Plural threadedpositioning rods 49 are telescopically slidable in bores in die 11. Eachrod 49 has a lower end 50 threadably secured to bar 47 and an upper end52 within a coaxial bore extension in bracket 54 fixed to side surface56 of die 11. Each rod 49 has an exposed threaded section along itslength. Exposed threaded nut 57 may be manually rotated with wrench 58along an associated threaded portion of each rod 49. When desired tolocally change (for example to increase) the cross sectional opening inpassage 40 beneath bar 47 relative to that of adjacent section(s) ofsuch passage, one or more nuts 57 (such as 57a in FIG. 5) bearingagainst surface 59 of die 11 is turned to forcibly draw rod 49aoutwardly, which, since rod 49a is secured at 60 to choke bar 47, causesbar 47 to bend outwardly within slot 48 to form passage region 62locally increased in cross sectional size relative to adjacent region64. The locally increased passage size may optionally be developed (orthat provided by choke bar 47 as just described may be augmented)forwardly of choke bar 47 in the direction of extrusion by bending apredetermined lateral zone of one or both slot-defining lips 32, 34outwardly relative to adjacent lip sections in generally the same manneras described for choke bar 47. This may be accomplished by manuallyforcibly deflecting such lip zone about a hinged area of decreasedthickness with an adjustable die lip control system, not shown. Movementof these members (i.e. bar 47 and local portions of the lips) in themanner described are on the order of a few thousandths of an inch.

Cutting means 22 (FIGS. 1 and 2) comprises an industrial robot,exemplarily illustrated at 66, having rotatable stub shaft 68 adapted tobe program controlled in rotative movement by a conventional computersystem, not shown. Robot arm 70 (FIG. 2) secured to shaft 68 pivotablysupports link arm 72 on its forward end at 74. Arm 72 carries downwardlydirected high pressure impingement cutting mechanism 78 which in workingposition faces the underlying path of forward movement of sheet 12through the system. Cutting mechanism 78 via hose 80 is operativelyassociated with a source (not shown) of high pressure (e.g. 40,000 psi)water adapted, as exemplarily shown by cutting stream 81, to impingewith sufficient force to penetrate through predetermined portions ofsheet 12 to form a blank 24. Alternative forms of cutting means 22 maybe used as long as functional to successively cyclically sever sectionsforming a blank 24 out of sheet 12 in a manner to be further described.

Conventional rotating winder assembly 84 (FIG. 3) synchronously linkedwith the drive means turning pull roll 20a is downstream of cuttingmeans 22 for maintaining tension in the sheet as it passes through thesystem and winding scrap sheet 82 on itself after blanks 24 have beencut from sheet 12.

The process for forming blanks 24 using apparatus 10 comprisessequential steps the first of which includes extruding plasticizedpolyvinyl butyral melt from die 11 by forcing such melt through slot 16where it is shaped into endless, distortable sheet 12 initially havingsubstantially parallel edges 88, 90. On exiting slot 16 the polymer ofsheet 12 is at elevated temperature as a result of energy input in theupstream extrusion system which melts and causes the polymer to flow.Sheet 12 preferably contains integral gradient color band 46 adjacentone edge 90 which is graduated in color intensity from relatively deepadjacent edge 90 fading to extinction at laterally inward cut off line94. Color band 46 is formed by feeding a colored secondary flow ofmolten plasticized polyvinyl butyral polymer to and through an extrusionslot in probe 42 at substantially the same temperature and viscosity asthat of the main flow of molten polymer in manifold 30 of extrusion die11. Both the main flow and colored secondary flow are simultaneouslyextruded at different velocities toward the outlet slot 16 such that alayer of colored melt is completely encapsulated in the main flow ofpolymer. Further details of such encapsulation and extrusion aredisclosed in U.S. Pat. No. 4,316,868 which as indicated is in partincorporated herein by reference.

Conventional techniques known to those skilled in the art may beemployed in association with the extrusion process to produce a roughsurface on one or both sides of the extruding sheet. These involve thespecification and control of one or more of the following: polymermolecular weight distribution, water content of the melt, melt and dieexit temperature, die exit geometry etc. Systems describing suchtechniques are disclosed in U.S. Pat. Nos. 2,904,844; 2,909,810;3,994,654; 4,575,540 and published European Application No. 0185,863.

Because of the increased mass flow rate of melt occurring as a result ofthe increased cross section of region 62 of passage 40 provided via thepreset positioning of choke bar 47 as previously described, integrallyformed zone 96 of sheet 12 is locally increased in thickness relative tothe laterally adjacent portion containing color band 46. Thus, meltforming zone 96 of sheet 12 to be stretched the most under the influenceof rolls 20 in a manner to be described, has an initial thickness afterextrusion on issuing from the die opening and before stretching which isgreater than that containing color band 46 along the opposite side ofthe sheet.

Immediately after (and conceivably just before) issuing from the openingat the forward end of slot 16, the pendant sheet is pulled intofrictional shaping engagement with a preselected portion of theperimeter of rotating frusto-conical pull roll 20a which is driving roll20b at the same rotative speed. In so doing the side of the sheetcontaining increased thickness zone 96 is stretched and drawn under theinfluence of enlarged diameter portions 99, 100 of rolls 20. Stretchingwhile the sheet is at elevated temperature commences essentially at thepoint of exit from slot 16 and continues during the brief intervalbefore and during contact with the surface of lower roll 20a. Stretchingoccurs because points slant height of the conical surface of roll 20awith which the distortable sheet is in contact are moving at differentvelocities and thus apply different forces to the sheet across itswidth. Thus, the enlarged diameter end of the cone turning at a greatersurface speed than the other end stretches and thins the initiallylocally thick zone 96 of the sheet to a greater degree than the portionof the sheet containing color band 46 in contact with the smaller end ofthe cone. The radius of curvature of the shaped distorted sheet iscontrolled and conveniently changed by adjusting the position of thesheet along the slant height of lower frusto-conical roll 20a. Theconstruction center of roll 20a is schematically shown as 101 in FIG. 2which represents the intersection of the axis of rolls 20a and 20b. Thecurvature of the sheet along the side of smaller radius for typicalcommercial windshields of complex curvature should be within a range of50-300 in (152-762 cm).

Thus, as extrusion through die 11 continues the side of the sheetcontaining increased thickness zone 96 is stretched more than the otherside containing color band 46 primarily under the influence of theenlarged diameter portion of roll 20a. This reshapes the sheet from aninitial configuration with substantially parallel edges on issuing fromslot 16 into an arcuate form (FIG. 2) extending in the forward directionof extrusion. This stretching pattern reduces the thickness of zone 96to that of the rest of the sheet to provide a substantially uniformthickness sheet 12 after passage through nip 97, possessing a differentradii along each edge, that of greater radius being along the sideformed from the greater thickness melt.

As shown in FIG. 4, by directing the sheet downwardly into cooling bath26, sheet 12 is quenched and reduced in temperature to at leastpartially set the polymer and promote retention of the roughenedsurfaces previously formed during extrusion prior to passage around roll20a and through nip 97.

After leaving contact with rolls 20 and emerging from bath 26, theshape-stable sheet may be directed off-line (not shown) and accumulated,for example by winding on a conical core having substantially the sameconical taper as roll 20a, for later unwinding and cutting into discreteblanks. Since the sheet has already been shaped and its gagedistribution is therefore substantially uniform, the off-line blanksformed with this embodiment should be equal in quality to those formedwith the in-line embodiment now to be described.

The shaped sheet is advanced through tension applied by winder assembly84 or equivalent to the cutting station preferably along a forwardlycurved arcuate path substantially corresponding in curvature to thearcuate contour of the shaped sheet. One or more conical rolls 102, 104may be provided as necessary for supporting the sheet during suchmovement.

While the sheet passes through the cutting station, cutting mechanism 78which is synchronously tuned in timed relation to the advancing movementof the sheet, traverses a path corresponding to the desired peripheralpattern of a blank 24. High pressure water cutting stream 81 issuingfrom such moving mechanism 78 sequentially impinges on and penetratesthrough sheet 12 to cut blank 24 therefrom, which then falls by gravitythrough an opening in an underlying support table (not shown) onto blankstack 109. More particularly, arcuately shaped quadrilateral sectionsare sequentially cut out of sheet 12 which each have a gradient colorband extending along one longitudinal side to form such blanks. Asapparent from FIG. 2, each severed section will have longitudinal curvededges 110, 112 generally parallel to the outer curved edges of thepreviously distorted sheet. The peripheral contour of each blank isdesigned to substantially exactly match that of the panel(s) with whichit will be later laminated to form a windshield of complex curvature.Alternatively, to simplify the on-line cutting step, straight-sidedblanks (e.g. trapezoidal-shaped) may be cut from the sheet on-line andthen further trimmed off-line by an auxiliary cutting means, not shown,to the final arcuately shaped quadrilaterial configuration.

Scrap web 82 having successive, large through-holes corresponding inprofile to those of the blanks is wound on itself (FIG. 3) by winderassembly 84 for later comminuting in conventional manner and eventualreuse as a portion of the feed to the upstream extrusion system.

The following working example is for illustration only and should not betaken in a limited sense.

EXAMPLE

Polyvinyl butyral polymer commercially available from Monsanto Companyas Butvar® resin was mixed with dihexyl adipate plasticizer (32.5 phr)in a high intensity batch mixer and charged continuously to a ventedworm extruder (32/1 L/D). Melt at about 400° F. (204° C.) and3,000-5,000 psi (20.67-34.45 MPa) at the extruder outlet was end fed at276 lbs (125 kg) per hr to a sheeting die of the type shown in FIG. 4,i.e. polymer at 635 psi (4.38 MPa) entered cylindrical manifold 30 in adirection parallel to axis 45. A 28 in (71 cm) wide sheet having asurface roughness of 45-65×10⁻⁵ in (114-165×10⁻⁵ cm) on each side issuedvertically downwardly from the die, such roughness measured after thesheet had passed through the cooling bath and the nip between rolls 20.Integrally formed in the sheet was an 8.25 in (20.96 cm) wide coloredgradient band measured in a direction perpendicular to that ofextrusion. To determine the difference in mass flow rate across the dieoutlet slot, the extruding sheet was temporarily manually pulled fromthe outlet and immediately introduced into a 55° F. (12.8° C.) subjacentwater bath and samples across the width of the sheet taken for thicknessmeasurement. As a result of the choke bar setting upstream of the dielips and the associated manual adjustment of the position of portions ofthe die lips, the mass flow rate of melt was greater on the side of thesheet opposite that containing the gradient band. This was evident in agradual progressive increase in thickness across the full width of thesheet starting from the side of least thickness adjacent the outer limitof the color band. The instantaneous average thickness of the sheetacross its full width measured off-line in samples collected at aparticular time during the run was 35.3±9 mils (0.09±0.02 cm), with thethickness along the side containing the gradient band being about 26mils (0.07 cm) and that adjacent the opposite clear side being about 43mils (0.11 cm). After thus measuring the thickness profile, the sheetwas redirected around a pair of rotating metal-surfaced frusto-conicalpull rolls immersed in and at substantially the same temperature as thewater bath immediately below the die. The lower roll was three ft (0.9m) long and had 10 in (25.4 cm) and 16.5 in (41.9 cm) diametersrespectively at its small and enlarged ends. The surface speed was 11 ft(3.4 m) per min at the small end of rotating roll 20 and 14 ft (4.3 m)per min at its enlarged end. The vertical distance between the lower endof the die lips and the upper level of the water was about 2 in (5.1cm). While melt continued to issue from the die slot, the greaterthickness clear side of the sheet engaged the faster moving cone surfaceportion while the gradient side was wound around the opposite slowermoving surface portion. The velocity gradient across the sheet becauseof the lower pulling action of the uneven diameter rolls was visuallynoticeable in the sheet at the exit of the die slot. The initialsubstantially vertically straight-sided sheet passing aroundfrustoconical roll 20a and through nip 97 was distorted in the mannerdescribed into a curved shape with a measured 60 in (152 cm) radius ofcurvature along the edge adjacent the gradient band and, though notmeasured, an estimated different, significantly greater radius ofcurvature along the other edge of about 60+28=88 in (223.5 cm). Thesedimensions approximate those typically required in an interlayer for acommercial windshield of complex curvature. Sections of the distortedsheet were manually cut crosswise of the direction of extrusion andthickness measured. The average thickness of the originally uneventhickness sheet described above, measured at a subsequent point in therun, was substantially uniform at 29.2±0.75 mils (0.074±0.002 cm). Thethickness and width of the curved sheet was found to be a function ofextrusion rate, rotative speed of the lower frusto-conical roll and theangle of the cone (slant height of the roll) relative to the position ofthe sheet on the cone.

The method of the invention is capable of providing a cut blank ofplasticized polyvinyl butyral shaped in general conformance with theshape of a windshield of complex curvature and which has high qualityperformance characteristics insofar as not significantly varying inthickness across its width. Since shaping occurs while the plastic issubstantially at elevated extrusion temperature and is thereforesubstantially stress-free, on reheating during laminating such blankshould be shrink-stable and remain dimensionally accurate. Moreover,additives to enhance sheet performance incorporated into the plasticformulation charged to the extruder should not flash out of the sheetsince post-forming during laminating can be avoided.

The preceding description is set forth for purposes of illustration onlyand is not to be taken in a limited sense. Various modifications andalterations will be readily suggested to persons skilled in the art. Itis intended therefore that the foregoing be considered as exemplary onlyand that the scope of the invention be ascertained from the scope of thefollowing claims.

We claim:
 1. A process for forming plasticized polyvinyl butyral sheetwhich comprises: continuously forcing plasticized polyvinyl butyral meltthrough a slot while incorporating a colored stream of plasticizedpolyvinyl butyral into the melt upstream of the slot to form a sheethaving a gradient color band extending along one side, the mass flowrate of melt through one region of the slot being greater than throughthe remainder of the slot; radially forwardly distorting the side of thesheet opposite the side containing the color band to form a curved shapeof substantially uniform thickness having a different radius ofcurvature along each edge while the melt continues to issue from theslot, the edge of greater radius of curvature being formed from theregion of the sheet formed from the melt issuing through the slot at thegreater mass flow rate and passing the sheet through a cooling bathafter issuing from the slot to set the plastic thereof.
 2. The processof claim 1 wherein the colored stream is incorporated into the meltwhich is to form the region of the sheet formed from the melt issuingfrom the slot at the lesser mass flow rate.
 3. The process of claim 1wherein the sheet is distorted by engagement with the conical surface ofa pull roll.
 4. The process of claim 3 wherein the sheet is at leastpartially engaged with said roll while within the cooling bath.
 5. Theprocess of claim 4 wherein the melt issuing from the slot graduallyincreases in thickness in a direction perpendicular to the melt flowtoward the edge of the sheet having the greater curvature afterdistortion.
 6. The process of claim 4 including the step of successivelycutting discrete interlayer sections out of the continuous curved sheetwhich have longitudinal curved edges substantially parallel to those ofthe continuous distorted sheet.
 7. The process of claim 6 whereincutting occurs as the sheet continues to advance forwardly.